1. Field of the Invention
This invention relates to a thermally processable plastic container for foodstuffs which are susceptible to oxidative deterioration and more particularly to such a container having a peelable, abuse resistant lid.
2. The Prior Art
In recent years the food packaging industry has shown intense interest in the concept of plastic container packed foods which among other advantages do not require freezing for their preservation and can therefore dispense with costly energy intensive refrigerated transportation and storage facilities. Much effort has gone into the development of a rigid or semi-rigid retortable or autoclavable plastic food containers which in addition to withstanding the rigors of sterilization and later reheating and provide gas and moisture barrier properties which are sufficient to adequately protect the contents during storage, can also be effectively hermetically sealed with a lid which is readily peelable from the container without tearing or rupturing of the packaging materials. With respect to the peelable opening feature, as a simple matter of convenience, it is desirable that the food container be operable by manually pulling the lid from the container instead of relying on a knife, scissors, or tear tab to open the package.
Among the materials which have been investigated for use in the manufacture of the food container just described are laminated films of polyolefins such as polyethylene, polypropylene and copolymers thereof such as ethylene-propylene copolymers. The permeability of the laminated polyolefin films to gases is lowered by the incorporation in the laminated film structure of an inner layer of a gas barrier film such as a vinylidene chloride polymer (e.g. saran) or a hydrolyzed ethylene/vinyl acetate copolymer. Containers are manufactured from these laminated films, to which the desired shape is imparted, for example, by thermoforming. The container bodies generally possess a base and a sidewall which extends upwards from this base, thus forming a type of tray or tube of cylindrical, truncated-cone, square, rectangular or even polygonal shape, depending on the nature of the article to be packaged.
The containers are hermetically sealed by means of a protective cover or lid. For this purpose, the sidewall of the container terminates at its upper end in a radially outwardly directed flat flange, the lid being placed flat on the polyolefin flange surface and sealed under high pressure (e.g. 60 psi) and high temperature (e.g. 300.degree. C.).
Hitherto, aluminum foils of at least 0.10 mil thickness or laminates consisting of such an aluminum foil and one or more thermoplastic films such as polyethylene or polypropylene have been used as lid materials. The aluminum foil surface to be heat sealed to the polyolefin flange is provided with a heat activatable bond promoting layer which allows the foil to be heat sealed to the flange. Adhesive coating layer compositions which have been studied for bonding the aluminum foil layer to the polyolefin flange include epoxy resins, ethylene-vinyl acetate copolymer and a carboxyl modified polypropylene such as the maleic anhydride adduct of an ethylene-propylene copolymer, one of which, Morprime (Morton Chemical, Division of Morton Norwich Products, Inc.) has been cleared for use in food packaging by the Federal Drug Administration and is commercially available.
Heating sealing of the adhesive coated aluminum foil lid to the polyolefin container flange is generally sufficient to ensure that the package remains hermetically sealed during normal handling and during transportation and storage.
Although heat sealing of the aluminum foil lid to the polyolefin container flange through the heat activatable bond promoting layer is highly effective for bonding the lid to the flange, it has been determined upon further experimentation, that the strength of the heat seal exceeds the tensile strength of the materials used for the fabrication of the lid so that when removal of the heat sealed lid is attempted, a portion of the lid adheres to the flange surface with the result that the surface of the flange in the heat seal area has an unesthetic, rough and jagged appearance which is undesirable from the standpoint of consumer appeal. Further, because of the high strength bond, the force required to effect removal of the closure is too high for widespread consumer acceptance.
If it is attempted to weaken the heat seal to obtain a hermetically sealed container in which the lid can be readily opened, or peeled easily from the container flange, there results a seal of insufficient mechanical strength so that the package can be accidentally opened when dropped from a height or when subjected to abuse during normal handling and shipment.
It is therefore an object of the invention to provide a peelable, hermetically sealed thermally processable polyolefin plastic food container.
It is a further object of the present invention to provide a hermetically sealed polyolefin food container which can be peelably sealed and which can withstand accidental opening when dropped from a height or subjected to mechanical abuse during handling and transportation.
According to the present invention there is provided a method of manufacturing a polyolefin container which can be hermetically sealed with a lid which can be peeled open, which method comprises interposing between the lid and the container surface to be sealed an intermediate layer comprising a mixture of a particulate filler and first and second polyolefin resins, the second polyolefin resin having a melt flow rate of at least 3 times greater than the melt flow rate of the first polyolefin resin and then applying heat and pressure to form a hermetic seal.
The combination of particulate filler and polyolefin resin materials of different melt flow rates produces an intermediate heat seal layer in which the cohesive strength of the intermediate layer is less than bond strength of the heat seal so that when it is attempted to separate the lid from the container, the cohesive failure of the intermediate layer permits the lid to be readily peeled away from the container surface to which it had been originally heat sealed.
In mechanical abuse tests, containers hermetically heat sealed in accordance with the method of the present invention and then retort processed were found to withstand accidental opening under conditions which satisfy United States Department of Agriculture (USDA) mechanical abuse specifications for food containers.